Visualization of normal-zone initiation and propagation using fluorescent paints

To establish a stability criterion for Yttrium barium copper oxide (YBCO)-coated conductors, it is necessary to clarify their transient thermal characteristics. Thermocouple thermometers, resistance thermometers, or the relation between the temperature and resistance of the stabilizer used for the conductors are generally employed for measuring their temperatures. However, these methods can only be employed to observe fixed-point data. Hence, it is difficult to observe the longitudinal and transversal distributions of temperatures in coated conductors. In this study, we adopt a method for the visualization of the temperature distribution of coated conductors by using fluorescent paints whose color changes with temperature. By adopting this method, we can measure the two-dimensional distribution of temperature in magnetic fields and at cryogenic temperatures. Therefore, local heat generation in the YBCO tapes can be observed with a higher spatial resolution and the uniformity in the critical current density of YBCO tapes can be identified. In this paper, we describe the developed thermal visualization system and the preliminary experimental results of the visualization of normal-zone initiation and propagation in YBCO sample strips. Further, we also report a part of the normal-zone initiation and propagation mechanism by electromagnetic field and heat transfer analyses by employing a three-dimensional finite element method (FEM).